1. Field of the Invention
The present invention relates to a heat treating apparatus used in the manufacturing of a semiconductor device, a liquid-crystal driving circuit board, and so on and, in particular, to the improvement of a reaction tube cooling system for this type of heat treating apparatus.
2. Description of the Related Art
As part of the process for manufacturing semiconductor devices, liquid crystal driving circuits, and so on, a vertical or a horizontal type heat treating apparatus is in widespread use for heating products, such as semiconductor wafers and circuits, requiring heat treatment, thereby to facilitate film formation, diffusion, oxidation, etching, and so on.
In the aforementioned heat treating apparatus, the reaction tube into which is placed those products to be heat treated, is constructed of a cylindrical tube made of, for example, quartz, and in a typical apparatus, has a cylindrical liner tube made of, for example, silicon carbide located around the outer periphery thereof. A heating element is wound in the form of a coil around the liner tube, and is itself surrounded by heat insulation.
In use, the interior of the reaction tube is initially heated to a predetermined treatment temperature--for example, several hundred to a thousand and several hundred of degrees--and products to be heat treated, such as the semiconductor wafers on a wafer boat, are loaded into the reaction tube through the lower opening thereof in the case of a vertical type apparatus, and through one end thereof in the case of a horizontal type apparatus. Then, a predetermined reaction gas, such as SiH.sub.4, O.sub.2, B.sub.2 H.sub.6, and PH.sub.3 is supplied into the reaction tube so as to form a film on the semiconductor wafer or to perform some other treatment, such as diffusion.
As products to be heat treated become increasingly sophisticated, it is necessary that the heat treating apparatus be provided with a cooling system which can rapidly and uniformly cool the entire reaction zone within the reaction tube, since the above cooling treatment is important in order to obtain a uniform thermal treatment history in the one or more semiconductor wafers being treated, as well as to reduce the process time.
Japanese Patent Publication (KOKOKU) 58-24711 discloses a cooling system used in a horizontal type heat treating apparatus. In this cooling system, a cooling pipe is arranged within the heat insulation surrounding the heating coil, resulting in a cooling function of low efficiency.
Japanese Patent Publication (KOKOKU) 60-8622 discloses another type of cooling system used in a horizontal type heat treating apparatus. In this system, a heater is directly located around the reaction tube and is enclosed in a heater casing, as a result of which a liner tube is not required. A cooling fluid is moved, by means of a blower, from one end of the reaction tube into a space between the outer surface thereof and the inner surface of the heater casing, and is exhausted through the other end of the tube. In the system as set forth above, the flow of cooling liquid along the reaction tube is, however, likely to deviate from a desired flow pattern, failing to cool the entire reaction zone within the reaction tube uniformly.
Japanese Utility Model Disclosure (KOKAI) 61-157325, on the other hand, discloses a cooling system used in a barrel type heat treating apparatus. In this cooling system, a heating coil is located inside the inner cylinder of a reaction tube on which a semiconductor wafer is supported by means of a susceptor. A cooling liquid is blown from the upper portion of the apparatus in a tangential direction of the outer cylinder of the reaction tube to allow it to flow along the outer surface of the outer cylinder of the reaction tube, and is exhausted through the lower portion of the apparatus. In this system, a cooling function is not so efficient, due to the lingering heat of the heating coil situated within the inner cylinder of the reaction tube. Furthermore, due to a greater temperature difference between both side surfaces of the semiconductor wafers, the semiconductor wafers might be deformed when they were cooled rapidly.